1. Field of the Invention
This invention relates to a fluorine-containing polyimide membrane for gas separation. More particularly, it is concerned with a composite or asymmetric membrane used for separating and concentrating a particular component from an industrial mixture of gases, a process for manufacturing the same, and a method of separating and concentrating a particular component from a mixture of gases using the same.
2. Description of the Prior Art
It is known that the polyimides make separation films or membranes having good heat- and chemical-resistant properties, as they have a high glass transition temperature and rigid structure. The known polyimides have, however, been unsatisfactory in permeability, though they exhibit high selectivity, as compared with other vitreous polymers.
Fluorine-containing polyimides have been found to exhibit high permeability, as compared with the prior polyimides, while retaining high selectivity, due to, the bulkiness of the hexafluoroisopropyl group, --C(CF.sub.3).sub.2 --, which they contain, and the improved solubility of gas by fluorocarbons. More specifically, there have, for example, been proposed homogeneous polyimide membranes composed of an aromatic acid anhydride including 5,5'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene-bis-1, 3-isobenzofurandione (6FDA), and an aromatic diamine, methylenedianiline, etc., as described in Japanese Patent Application Laid-Open No. 123420/1988, U.S. Pat. No. 4,705,540, Japanese Patent Application Laid-Open No. 262925/1989 and Japanese Patent Application Laid-Open No. 261524/1990.
These homogeneous polyimide membranes have, however, been still unsatisfactory in permeability, selectivity, and applicability to different kinds of gases. Moreover, the mechanical strength required of those membranes have made them too large in thickness to exhibit any permeation rate that is satisfactory for practical and industrial use.